Maybe I'm missing something obvious, but whenever I try to render an 
image with a transparent background, any emissive media that does not 
have a solid object between it and the background just becomes 
invisible, instead of partially transparent which is the intent.

Essentially, I'm trying to render an engine glow via emission, and it 
works fine when I have a backdrop, but it disappears when I remove the 
background.

minimal scene:
sphere{0,1
   texture{pigment{color rgbt 1}}
   hollow
   interior{
     media{
       emission <1,0,0>
       method 3
       intervals 1
       density{color rgb 0.5}
     }
   }
}
camera{location <0,2,-2> look_at <0,0,0>}

Setting -UA in the command line shows a red sphere, +UA is completely 
transparent. Upon opening in photoshop, it shows that the red channel is 
in fact correct, but the alpha channel is at 100% for the entire image.

Is there a way to get around this?

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